Waterjet propulsors have been available for many years. Some of their obvious advantages over propellers include that they have no exposed rotor, have low underwater noise signature, offer even engine loading, and offer shallow draft. However, the waterjet's efficiency falls far short of the efficiency of an underwater propeller at low boat speeds. The propulsive coefficient of a typical underwater propeller at 16 knots is about 65 percent while that of a waterjet at the same 16 knots would be only about 40 percent. Those numbers given an advantage to the underwater propeller of 38 percent at that 16 knot speed. The waterjet becomes more competitive at higher speeds where the drag of the underwater propeller's appendages including shaft, strut, rudder, etc. causes it to have a severe disadvantage. The competition to the waterjet then becomes the surface propeller that, in its normal design, operates aft of the transom or a step in the boat's bottom. Only the lower half of the surface propeller is in the water. As such the surface propeller avoids shaft, thrust, and, in some designs, rudder drag. While generally considered to be rather inefficient at low boat speeds, the surface propeller is considered the favored propulsor at very high boat speeds.
The Kort Nozzle, first introduced in the 1930's, yields even greater performance for a variation of the propeller at low boat speeds. It applies a simple ringed nozzle around the periphery of an underwater propeller. By use of carefully designed angled airfoil shapes to the nozzle ring it is possible for the Kort nozzle to actually gain thrust from external forces acting on the nozzle. A well designed Kort nozzle shows noticeable performance gains over a standard underwater propeller at speeds up to, say, 16-20 knots. Beyond those speeds, the drag of the nozzle itself rules out use of the Kort nozzles. As such, Kort nozzles are widely applied to tug boats and other low speed mostly work boats. For purposes of this application, low speed is defined as boat speeds up to and including 20 knots and high speed as boat speeds of over 20 knots.
In summary, the waterjet propulsor is severely outclassed from efficiency standpoints at low to moderate, up to about 25 knot, and very high, over 60 knot, speeds. The reason for much of its efficiency shortcomings has to do with its inlet performance. A well-designed waterjet pump can have a rotor efficiency of 93 percent, flow straightening stator vane efficiency of 92 percent, and discharge nozzle efficiency of 98 percent. That comes to an overall pump efficiency of 84 percent. However, its averaged inlet pressure recovery efficiency will probably only be in the 70 percent area. Consequently, the best overall efficiency that can be expected from such a waterjet propulsor while running at its best performance at hihg boat speeds is about 59 percent. The major reason that waterjet inlet efficiency or inlet pressure recovery is so poor is because of distortion in the inlet flow. The high velocity incoming water in a typical flush with the hull waterjet inlet piles up over the lower half of the inlet duct. Due to this distorted flow, the rotor generally sees recoveries of 90 percent or more of boat freestream dynamic head over its lower half and as low as 50-60 percent over its upper half.
The instant invention offers greatly improved thrust values for the waterjet at all boat speeds. In its preferred embodiment, it decreases the amount of flow distortion that the rotor sees as well as gains thrust advantage from an airfoil shaped flow deflector strategically placed in the inlet duct. Further, generally airfoil shaping of the lower outside portion of the inlet housing so that such housing is submerged adds to thrust with little or no increase in external drag.
In addition to the significant performance gains in waterjet performance to be realized by the instant invention, a significant advantage in the form of a discharge jet-trimming device is also offered. These features are described in detail in the following sections.